* Upgrade xDS (go-control-plane) API to support Envoy 1.10.
This includes backwards compatibility shim to work around the ext_authz package rename in 1.10.
It also adds integration test support in CI for 1.10.0.
* Fix go vet complaints
* go mod vendor
* Update Envoy version info in docs
* Update website/source/docs/connect/proxies/envoy.md
* Refactors the HTTP listen path to create servers in the same spot.
* Adds HTTP/2 support to Consul's HTTPS server.
* Vendors Go HTTP/2 library and associated deps.